1. Field of the Invention
The present invention relates to electrical energy storage systems and, in particular, to battery packs composed of multiple battery modules with adjustable configurations.
2. Description of the Related Art
Hybrid electric vehicles (HEVs), hybrid electric ships and boats (such as the QE2) afford greater fuel efficiency than vehicles or vessels having only a prime mover (e.g., diesel or gasoline engine, gas turbine and fuel cell). Greater efficiency is obtained by using an energy store to level the load on the prime mover-providing peaking power to an electric motor, or storing energy during low power prime mover operation or during regenerative braking.
A well-developed form of energy storage for this application is a battery pack, and there are several candidate battery chemistry systems that may be utilized. Of these systems, the lithium-ion (Li-ion) technology is the most energy and power dense. Li-ion cell technology for this application is currently capable of energy density of up to 134 W-hr (482 kJ) per kg (HE40 cell), and a power density of up to 13 kW per kg (e.g., HP18650 cell short duration, manufactured by SAFT AMERICA).
Vehicle battery packs sized for multiple military combat missions, (such as the Combat Hybrid Power Systems (CHPS) Systems Integration Lab (SIL) battery pack) store approximately 108 MJ (30 kW-hr). If the highest power density SAFT cells (HP18650) are utilized, then the short duration power capability of the battery pack would be greater than 4 MW.
Such a peak power capability could enhance both offensive and defensive capabilities if it were easily convertible to voltages commensurate with a range of potential short-term and pulsed loads. For example, Electromagnetic Armor (EMA) provides lightweight protection to combat vehicles against rocket-propelled grenade attacks. The energy required for the EMA to function is stored in a fast discharge capacitor bank, which is recharged either from a generator operated by the prime mover of the vehicle, or from an intermediate energy storage system. In a HEV, the electric energy battery pack also could function as such an intermediate energy storage system. However, in order to provide the approximately 10 kV needed as the input voltage to the capacitor bank, a DC-DC power converter is needed between the intermediate energy storage system and the capacitor bank.
The traditional methods for DC-DC power conversion typically involve the use of inverters and heavy-duty transformers. Current EMA designs require approximately 150 kJ to recharge the capacitor bank. At 3 pulses per second (pps) operation, even for a short period of time, the recharge time would have to be on the order of 300 ms. Consequently, the recharge power would have to be on the order of 500 kW average, or 1 MW peak. However, a 1 MW peak power traditional DC-DC converter, using present technology, would add more volume and mass to the vehicle platform than is allowable in a 16-20 ton class vehicle.
Thus, there exists a need in the art for a system power density improvement in producing short term high power at high voltages from a battery pack normally configured to provide hybrid vehicle prime mover load leveling and/or silent mobility capability or in other fixed or moving battery platforms such as battery backpack systems or pulsed energy weapons or launch systems.